1. Field of the Invention
This invention relates to a process for producing particulate novolac resins, and the novolac resins so produced. The production of stable aqueous dispersions of novolac resins is also provided.
2. Description of the Prior Art
The prior art has disclosed procedures for preparing stable aqueous dispersions of heat-hardenable, resole phenolic resins, such as in U.S. Pat. No. 3,823,103 (Harding). Procedures have also been utilized for the preparation of particulate resole resins isolated from aqueous dispersions, such as those discribed in U.S. Pat. No. 4,206,095 (Wynstra et al.). These processes produced resole resins more efficiently and safely than other prior art processes. Also, since the product is in the particulate state, intensive grinding is not required.
It would be desirable if a similar procedure could be developed for the preparation of particulate novolac resins, as opposed to the production of particulate resole resins as described in the prior art. There exists, however, significant differences between the two basic types of phenolic resins, i.e., novolacs and resoles, as are well known to those skilled in the art. If the phenolic resins are prepared with an excess of formaldehyde and an alkaline catalyst, the product will possess methylol side or end groups, and can be referred to as resoles or one-stage phenolic resins. The resole resins can be cured with heat through the condensation of the methylol groups, without the addition of curing agents. If, however, the phenolic resins are prepared with an acidic catalyst and less than a mole of formaldehyde per mole of phenol is utilized, the resin will be a phenol-ended chain polymer in which the phenolic groups are connected with methylene bridges located ortho and para to phenolic hydroxyl groups, will have the properties of being permanently soluble and fusible, and will cure only upon the addition of a curing agent. Since the addition of a curing agent is required, these novolac phenolic resins have been referred to as two-step resins. These differences in chemical structure, production methods, and physical properties make it difficult to predict whether certain procedures effective for the one type of resin, would be effective when applied to the other type.
Utilizing the procedures described for the production of particulate resoles, it was initially believed that producing and isolating novolac resins in particulate form from a suspension polymerization would be easier for the novolac than for a resole. This assumption is based on the fact that novolac resins have a higher glass transition temperature, and are therefore more sinter resistant. The novolac resins are also not heat reactive, as opposed to resole resins, such that the drying procedure should be less critical. In addition, it was believed that the novolac resins should be more hydrophobic than the resole resins since the novolac resin is higher in molecular weight and does not contain methylol groups. It was discovered, however, that some of these assumption are incorrect. For example, novolac resins were found to be more hydrophilic than the resoles produced by the particulate process. As a result, the novlac resin particles in aqueous dispersion would require the special handling as with the particulate resoles. It was also discovered that since the molecular weight of the novolac was higher than the resiole, the tendency to form fine particles was actually less because of the higher melt viscosity. It would, therefore, be desirable if a procedure could be worked out which overcomes these difficulties to thereby produce a viable, particulate novolac resin.